In recent years, in order to suppress global warming, reduction of carbon dioxide generated in various fields has been demanded. For example, in the automotive industry, a shift from conventional gasoline vehicles to electric vehicles and hybrid vehicles that are equipped with a secondary battery with less carbon dioxide emissions is expanding, and, in particular, development of lithium ion secondary battery that affects the mileage, safety and reliability has been attracting attention. In general, such a lithium ion secondary battery is configured from a non-aqueous electrolytic solution, a separator, an external packaging material, or the like, in addition to a cathode including a cathode active material layer formed on a cathode current collector and an anode including an anode active material layer formed on an anode current collector.
Conventionally, in the common lithium ion secondary batteries, a transition metal oxide containing lithium as a cathode active material has been used, and also the cathode active material has been formed on an aluminum foil serving as a cathode current collector. In addition, a carbon material such as graphite is used as the anode active material, and the anode active material is formed on a copper foil serving as an anode current collector. The cathode and anode are arranged via a separator in a non-aqueous electrolytic solution containing an organic solvent in which lithium salt electrolyte has been dissolved.
In the charging and discharging of the lithium ion secondary battery, lithium ions stored in the cathode active material are de-intercalated and released into the electrolytic solution during charging, and in the anode active material, a reaction is allowed to proceed due to the occlusion of lithium ions from the electrolytic solution between the crystal layers of the carbon material. In addition, a reaction opposite to that during charging is allowed to proceed at the time of discharging, and the reaction proceeds due to lithium ions being released from the anode active material and occluded in the cathode active material.
However, in a system using a carbon material such as graphite for an anode, when the discharge is close to 100%, the potential of the anode approaches 0 V, resulting in the deposition of dendrite. As a result, the lithium ions that are originally used in the electron transport are consumed, and it would further corrode and deteriorate the anode current collector. If such corrosion and deterioration progress, they may cause deterioration in the characteristics as the lithium ion secondary battery and failure. For this reason, in the system using a carbon material such as graphite for an anode, precise control of the charge and discharge voltage is required. In such systems, even when the potential difference between the cathode active material and the anode active material is large in theory, only a portion of lithium ions can be used, and there was a problem in terms of charge and discharge efficiency.
Accordingly, in recent years, research and development of the anode active material in which a high potential can be achieved have particularly been carried out actively. For example, since titanium dioxide has an electrical potential of about 1.5 V which is higher than the electrical potential of the conventional carbon material, it has been gaining attention as a material which does not cause the deposition of dendrite, and which is very safe and also capable of achieving high performance.
For example, in Patent Document 1, a secondary battery obtained by spray drying a slurry containing a hydrous titanium oxide and heating an organic binder for removal which uses titanium oxide with a void volume of the secondary particles of 0.005 to 1.0 cm3/g as an electrode active material has been described.
In addition, in recent years, it has been reported that titanium dioxide having a crystal structure of bronze type is also promising as an anode active material. For example, in Patent Document 2, a secondary battery using a bronze-type titanium dioxide having an isotropic shape of micron size for the electrode active material has been described.
In general, titanium dioxide has been used in a wide range of fields, including white pigments, dielectric materials and photocatalyst materials, and is widely circulated as a very inexpensive material. However, when titanium dioxide is used as it is as an anode active material of a lithium ion secondary battery, although the safety of the secondary battery can be improved, there is a problem in that the electrical capacitance is as small as about 160 mAh/g.
For example, although the secondary battery described in Patent Document 1 exhibits favorable cycle characteristics, since the electrical capacitance is as small as 160 mAh/g, it is necessary to use a large amount of anode active material in order to obtain a predetermined battery capacity. For this reason, there is a problem in that the weight and volume of the battery as a whole become large with the secondary battery described in Patent Document 1.
In addition, the secondary battery using a bronze-type titanium dioxide as the electrode active material like the one described in Patent Document 2 is facing a host of challenges for the practical use thereof, since the electrical capacitance is as small as 170 mAh/g and also the process is complex and requires a prolonged period of time.